Loader wont hold position

[Wayne County Hose]

yes... double acting cylinder wont drop down unless valve is leaking....here is a clue.....the volume of the cylinder rod goes where???!!!

Sorry AKKAMAAN, but this is just not true. I do know the theory behind what you say. However, I have had hundreds of cylinders brought into my shop. People complaining that the cylinder drifts. I explain the same thing everytime. 99% chance the piston seals are worn and resealing the cylinder will solve the problem. Outside chance the valve spool or other component is worn causing internal leakage. I rebuild the cylinder and what do you know, customer comes back and says that the problem is solved. Ask absolutely anyone in a hydraulic shop and you get the same answer. I'm no engineer or scientist, just know what I see in the real world.

 

[beenthere]

I guess the oil just moves past the seals from one side to the other...:confused3:

 

[AKKAMAAN]

The 420 loader on my JD 4200 started dropping a little as it it has air in the lines. It will drop about six inches then stop. This started suddenly this week. I changed the fluid, filter & cleaned suction screen last fall. I switched the lines around so the curl circuit raises and lowers the loader....same thing. It will sometimes lift very slowly until it gets about half way up then speed up. Anybody have an idea as to what's going on?

M.D.

I have a question that I should have asked earlier....
Do you have POWER DOWN and FLOAT on that FEL, in addition to the LIFT function...or just lift and lower by gravity???
With float it would take a four position valve though...

 

[AKKAMAAN]

Sorry AKKAMAAN, but this is just not true. I do know the theory behind what you say. However, I have had hundreds of cylinders brought into my shop. People complaining that the cylinder drifts. I explain the same thing everytime. 99% chance the piston seals are worn and resealing the cylinder will solve the problem. Outside chance the valve spool or other component is worn causing internal leakage. I rebuild the cylinder and what do you know, customer comes back and says that the problem is solved. Ask absolutely anyone in a hydraulic shop and you get the same answer. I'm no engineer or scientist, just know what I see in the real world.

I do Agree with what you say about the piston seal. I have not said that Piston seal wont be bad. What I am saying is that The valve MUST leak as well....Otherwise the cylinder wont go down!!

This might be hard to understand, but the volume difference between capped side and rod side of piston, equals the volume of the rod. And when cylinder drops, rod goes into the cylinder....if seal is bad, pressure on capped side and rod side will equalize ae the cross area of the rod is the net piston area...and to drop down the volume of the rod need somewhere to go....and that will be out through the valve.......and if this valve have a spool for single action (even though the cylinder is a double action), the oil will leak out through the wide open rod side drain workport.

If cylinder is true double acting, the oil volume of the rod, MUST go out (leakage) trough one or both work ports (spool valve lands). (the only option to that is external leakage).

To remember is that if we have a heavy load in the bucket, which create a for example, 2500psi when cylinder seal is OK, not leaking. If the rod diameter is half of the bore diameter, the pressure will be 10000psi, under that load, when cylinder seal is leaking....10000psi will almost make hoses burst, and for sure some leakage even trough the best spool valve

math
3.0" bore x-area=7.0686 force 17672lbs >> pressure 2500psi

1.5" rod x-area=1.7671 force 17672lbs >> pressure 10000psi

This can also be reviewed at Brendan Casey's Troubleshooting hydraulic cylinder drift

 

[Wayne County Hose]

I completely undersand everything you discuss. However, this theory absolutely does not hold true in the real world. Also, I have zero respect for Mr. Casey and his ramblings. He is simply the self proclaimed king of internet hydraulics. He also tells you to remove your suction strainer and throw it away.

Look, don't and won't get into a peeing match over this. Just simply stating that a worn piston seal, in the real world not on paper, will absolutely cause a cylinder to drift with no other leakages, internal or external. Simple indisputable fact. I can give you the name and phone #'s of loads of my customers that will swear to it.

 

[AKKAMAAN]

I completely undersand everything you discuss. However, this theory absolutely does not hold true in the real world. Also, I have zero respect for Mr. Casey and his ramblings. He is simply the self proclaimed king of internet hydraulics. He also tells you to remove your suction strainer and throw it away.

Look, don't and won't get into a peeing match over this. Just simply stating that a worn piston seal, in the real world not on paper, will absolutely cause a cylinder to drift with no other leakages, internal or external. Simple indisputable fact. I can give you the name and phone #'s of loads of my customers that will swear to it.

I do not like to post this, because I know you will hate me even more ........but I do it for the others, that are more patient with my posts....:thumbsup:

I totally agree that most of the times (real world) a blown cylinder seal causes cylinder to drift, especially under load pressure, BUT IT WILL NEED HELP. A real bad seal is like taking the whole piston of the rod.....and all suddenly we have a single action cylinder with the "bore" size of the rod.....If cylinder drifts, where is the oil going out???
If no external leakage, or work port relief valve.....the spool is the only option!!! PERIOD!!!

Now we have a few spool options that can cause the drift!!!

1> Natural leakage, designed by valve manufacturer, this wont make cylinder drift very fast though....but faster at 10000psi than designed 2500psi....

2>The spool is designed with constant drainage to tank on port connected to rod side of this "double acting" cylinder (when cylinder only can get lowered by gravity)

3>The spool have some wear from long time operation and the spool leakage is increased....will create (much) faster cylinder drift than point 1>

PS
The reason why the cylinder seal is so common cause for drifting, is that the seal can break down instantly, but the spool need 1000's of hours operation time before it is worned enough cause drifting by it self. But together with a leaking seal they both need each other s leakage to cause the drift

 

[Mighty Dodge]

Wow, this thread livened up. I ordered the cylinder rebuild kits today. I'll let everyone know what happens after the rebuild.

Thanx for all the info

M.D.

 

[kennyd]

Wow, this thread livened up. I ordered the cylinder rebuild kits today. I'll let everyone know what happens after the rebuild.

Thanx for all the info

M.D.

I am just down the road a piece if you get in a bind

 

[AKKAMAAN]

Wow, this thread livened up. I ordered the cylinder rebuild kits today. I'll let everyone know what happens after the rebuild.

Thanx for all the info

M.D.

I am sure the cylinder is going to be fine after a new seal kit, but as I posted earlier, depending on what kind of spool you have on the rod side port, there could be an valve issue as well!!

Do you have force down on the loader??? Or just power up and gravity down??

 

[Mighty Dodge]

Yes it has down pressure.